The invention disclosed and claimed herein pertains generally to the field of acoustic signal source detection by means of a beam-forming array comprising N(N&gt;3) discrete acoustic sensor elements. More particularly, the invention pertains to such detection wherein a self-cohering method is used to locate the respective positions of the elements in an environment, that is, to a method which employs signals sensed by the sensor elements themselves for sensor position location. Even more particularly, the invention pertains to a self-cohering method of sensor position location which relies on the processing of one or more signals present in the background acoustic field of environment.
Arrays of acoustic sensor elements, such as hydrophones or the like, are extensively used in an acoustic environment such as an ocean body to locate the position or bearing of an acoustic signal source contained therein, and to provide signal to noise gain in an acoustic detection system. Such arrays generally comprise a number of sensor elements. The reception capabilities of individual sensor elements together form a beam or search pattern. Beam forming, the processing of signals to determine the direction or bearing of a signal source from a beam-forming array, and the provision of signal-to-noise gain, is generally based on two assumptions. The first is that all of the sensor elements of the array are known or measured with a separate element location system. The second is that acoustic signals generated by the signal source are always plane waves which have minimal phase distortion over the array.
Great mechanical or electrical simplicity is achieved if the elements are allowed to move and no element location system is required.
In order to determine the actual positions of the sensors at present, a high-frequency pinger or the like may be located in proximity to the array, and the time delays in pinger signal detection may be compared for each of the elements. However, the use of such active signal devices may be undesirable in military applications of a beam-forming array, where a hostile observer may be able to determine array location from the pinger signals. The undesirability is magnified if the array is continuously changing shape, requiring continuous operation of the pinger.
The present invention provides a self-cohering method for continuously determining the actual positions of each of the N sensor elements in an arbitrary beam-forming array to within a preselected error, such as to within one tenth of the wavelength of a received acoustic signal. The method is entirely passive, i.e., it does not require an active signal projecting device in proximity to the array. The invention thereby provides a solution for one of the present major problems in the art.